[sailfish-browser] Decrease tab page loading time

- TabPage is precompiled at BrowserPage
- TabPage's pull down menu is moved behind an async loader
- Radial gradient of TabItem is replaced by graphic asset + colorization
- TextField is moved behind an async loader
- Model (favorites, history, tabs) are set as late as possible to defer unnecessary delegate creation
- Busy indicator is shown for a brief moment while loading tabs grid
- Small improvements  for FavoriteList and HistoryList