Added link to Launchpad snapshot PPA to README.md

(patched from the snapshots branch)
Added button to open download folder. Fixes #21.

Also added tooltips to folder buttons.
Fixed showing 0% download progress
Fixed progress bar to actually show unknown progress.

Issue remains: drawing black text even on dark background.
Translated URL edit context menu into Finnish
Renamed Finnish translation file as locale name -> more automatic.

Added built-in translator. Related to #24.
Fall back to fi.ts, it works even if locale name is fi_FI
Added language selection and main menu. Fixes #24.
Refactored VideoTableModel usage at MainWindow
QT 5 fixes.
Additional gitignores for Windows.
(fix compiler warning)