[BrowserWindow] Enable single key shortcuts

Translate single keys to shortcuts:

1 - previous tab
2 - next tab
(basic Opera compatibility)

There are more to choose from, but 1 and 2 are probably the most used.
Full list:
http://help.opera.com/Windows/9.50/en/keyboard.html#single-key

Configurable in preferences/Keyboard shortcuts, off by default.

Closes #1172
Merge pull request #1282 from kdave/single-key-shortcut-v2

[BrowserWindow] Enable single key shortcuts
[WebTab] Don't show "Loading.." on empty tabs

[ci skip]
[TabWidget] Make sure current tab is loaded on session restore

Fixes issue with current tab not being loaded on session restore
with "don't load tabs until selected" option.
It was needed to switch to other tab and then switch back to first tab
to actually load it.

Also pause updates of the whole browser window when restoring session.
[Build] Fix building with Qt 4.7
[Click2Flash] Fix playing YouTube videos with Qt5

Closes #1283