[Preferences] Lazy reload of MainApp preferences

Re-setting user stylesheet with a lot of AdBlock hiding rules
takes quite some time for QtWebKitSettings.
[QupZillaPages] Always enable JavaScript for qupzilla: pages

Closes #1150
[Preferences] Network cache value can now be 20-200 MB

Increase the maximum cache size. See #1139
[CloseDialog] Fixed string in window close dialog.

Are you sure to quit QupZilla -> Are you sure to close this window
[KWalletPasswords] Build only with Qt4.

Also show better error message when loading plugin fails
[AdBlockDialog] Options button is now QPushButton

Qt renders better down icon indicating the button has menu
[Buttons] Use QPushButton for buttons with menu everywhere