[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
[Preferences] Update help text for single key shortcuts

Put the list of implemented shortcuts into a tooltip with a bit of
formatting.
Merge pull request #1284 from kdave/prefs-help-singlekey

[Preferences] Update help text for single key shortcuts
[BrowserWindow] Add slash as a single key shortcut for search
Merge pull request #1285 from kdave/singlekey-search

[BrowserWindow] Add slash as a single key shortcut for search