Added material icons from CC licensed "Google material icons" set.

Fixes #240.

To make this easier, I added a script to aid in downloading icons.
Checkout F-Droid/tools/download-material-icon.sh for more details.

The icons are licensed under the CCv4 attribution license, which I
added a shout out to under "License" in the README.md.

F-Droid/res/menu/main.xml

@@ -4,12 +4,12 @@
 
     <item
         android:id="@+id/action_search"
-        android:icon="@android:drawable/ic_menu_search"
+        android:icon="@drawable/ic_search_white"
         android:title="@string/menu_search"
         app:showAsAction="always"/>
     <item
         android:id="@+id/action_update_repo"
-        android:icon="@drawable/ic_menu_refresh"
+        android:icon="@drawable/ic_refresh_white"
         android:title="@string/menu_update_repo"
         app:showAsAction="ifRoom"/>
     <item
@@ -18,22 +18,22 @@
         app:showAsAction="ifRoom"/>
     <item
         android:id="@+id/action_manage_repos"
-        android:icon="@android:drawable/ic_menu_agenda"
+        android:icon="@drawable/ic_view_headline_white"
         android:title="@string/menu_manage"
         app:showAsAction="ifRoom"/>
     <item
         android:id="@+id/action_bluetooth_apk"
-        android:icon="@android:drawable/stat_sys_data_bluetooth"
+        android:icon="@drawable/ic_bluetooth_white"
         android:title="@string/menu_send_apk_bt"
         app:showAsAction="ifRoom"/>
     <item
         android:id="@+id/action_settings"
-        android:icon="@android:drawable/ic_menu_preferences"
+        android:icon="@drawable/ic_settings_white"
         android:title="@string/menu_preferences"
         app:showAsAction="ifRoom"/>
     <item
         android:id="@+id/action_about"
-        android:icon="@android:drawable/ic_menu_help"
+        android:icon="@drawable/ic_help_white"
         android:title="@string/menu_about"
         app:showAsAction="ifRoom"/>
 

F-Droid/res/menu/manage_repo_context.xml

@@ -5,11 +5,11 @@
     <item
         android:id="@+id/edit_repo"
         android:title="@string/edit"
-        android:icon="@android:drawable/ic_menu_edit" />
+        android:icon="@drawable/ic_edit_white" />
 
     <item
         android:id="@+id/delete_repo"
         android:title="@string/delete"
-        android:icon="@android:drawable/ic_menu_delete" />
+        android:icon="@drawable/ic_delete_white" />
 
 </menu>

F-Droid/res/menu/manage_repos.xml

@@ -4,12 +4,12 @@
 
     <item
         android:id="@+id/action_update_repo"
-        android:icon="@drawable/ic_menu_refresh"
+        android:icon="@drawable/ic_refresh_white"
         android:title="@string/menu_update_repo"
         app:showAsAction="always|withText"/>
     <item
         android:id="@+id/action_add_repo"
-        android:icon="@android:drawable/ic_menu_add"
+        android:icon="@drawable/ic_add_white"
         android:title="@string/menu_add_repo"
         app:showAsAction="always|withText"/>
     <item

F-Droid/res/menu/swap_next_search.xml

@@ -7,7 +7,7 @@
 
     <item
         android:id="@+id/action_search"
-        android:icon="@android:drawable/ic_menu_search"
+        android:icon="@drawable/ic_search_white"
         android:title="@string/menu_search"
         android:titleCondensed="@string/menu_search"/>