WIP: Support different themes for the UI.

Start by supporting the existing "Theme.AppCompat" and
"Theme.AppCompat.Light.DarkActionBar" themes.

- Implement a preference for the user's choice of theme ("squeezer.theme")
  and a ListPreference in the UI to show it to the user.

- Update BaseActivity to deal with theme changes at runtime.

- Add drawables for holo light versions of the AV player controls.

- Use styleable attributes to refer to the drawables, and update everything
  that referred to the drawable to use the attribute instead, so that the
  the drawable appropriate for the theme is used.
Ensure the actionbar spinner drop downs follow the applied theme.

Prior to this change they had the incorrect colour on the light +
dark action bar theme.

- Adjust Util.getSpinnerItemView and getActionBarSpinnerItemView to
  take a Context parameter instead of an Activity. Use the context to
  fetch the layout inflater. This ensures the layout inflater uses the
  correct theme.

- Pass the ArrayAdapter's context as the parameter for the actionbar.

As the ArrayAdapter's context comes from actionBar.getThemedContext()
this ensures that the correct theme is used.
Refactor theme management code for ease of use with other activities.

- Move the code in to its own ThemeManager class.

- Call the ThemeManager from SettingsActivity so it follows the theme.
Style the current playlist list correctly.

- Highlight the currently playing item using abc_light_pressed_*
  drawables from appcompat.v7.

- Use the normal android background colour for un-highlighted items.

This uses the correct colours for the selected style.
Move the Theme enum and default values into the ThemeManager.
Ensure the text on the volume display panel is legible.

As the panel always has a dark background ensure that the text colour
is Android's primary/secondary colours for dark backgrounds.

Without this the theme-specific colours are chosen, and when the theme
is Light.DarkActionBar the theme colour is black/dark-grey.
Use gradle 2.1, and version 0.14.0 of the Gradle plugin.

(cherry picked from commit 9adfd8ba1f7e0d07116bf36beae0bc7b2d9eed6c)

Conflicts:
	Squeezer/build.gradle
Merge branch 'theme-support' into develop

Conflicts:
	Squeezer/build.gradle
	Squeezer/src/main/java/uk/org/ngo/squeezer/Util.java
	Squeezer/src/main/java/uk/org/ngo/squeezer/framework/BaseActivity.java