Fixing TEDtalks subtitle downloader

the introDuration parameter on tedtalks page has changed a little, now the
parameter appears using htmlentities, so I had to change the split for it to be
able to find the subtitles.
Also now the subtitles appears in seconds.
And I've provided 15 as a default value, since it's a good guess for modern
tedtalks (older ones have longer introductions)
Vacuum minidb-based databases too (bug 1721)
Pipe: Avoid downloading podcast cover art
Merge pull request #30 from daneoshiga/ted_subtitles_fix

Fixing TEDtalks subtitle downloader
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