Improvements to the extension system

- Add category metadata in every extension
- Show this category in the extension list gui
- Add "mandatory_in" and "disable-in" configuration for an extension
- Add Ubuntu unity check to enable/disable unity specific extensions
- Move "gpodder.win32" and "gpodder.osx" setting to the "gpodder.ui" namespace to be able to use it in the extensions category settings
- Only show metadata information in the right-click dialog of an extension
Dont warn about system files in windows (bug 1712)
Merge pull request #29 from brot/master

Improvements to the extension system
Merge pull request #31 from smunkel/master

Dont warn about unknown file if it is a system file (Bug 1712)
gpodder.util: Use gpodder.ui.win32 for new code
Add disable_in=win32 to a few extensions (bug 1729)
Merge pull request #33 from brot/bug1729

Add disable_in=win32 to a few extensions (bug 1729)
QML UI: Cleanup and simplify episode list
Initial QML Performance Changes
Some more tweaks to the QML UI
Episode loading animation
QML UI: Avoid crashes with zero refcounts
QML UI: Multi-Episodes Sheet should fill screen
QML UI: Only show Episode Actions with listview
QML UI: Show download feedback immediately
QML UI: Show subscribe list in Sheet
QML UI: Pull down to refresh

This avoids issues when the user clicks on the back
button and accidentally hits the refresh button on
the start page. Hope this is discoverable enough.
Add flattr option to the episode context menu
Merge pull request #34 from brot/flattr_context

Add flattr option to the episode context menu
Add smart grouping of extension context menu

Context menues from extensions were grouped by string parsing.
If "/" is in the string the string before the seperator is used as
root menu entry and the string after the seperator is used as
sub-menu entry