FE/Qt: Precompiled header hacking (work in progress).

FE/Qt: file header cleanups.

FE/Qt: Reworking empty-file selector used in Export/Import appliance wizards to use tool-button instead of push-one.

FE/Qt: Wizards: Import appliance: Get rid of QLabel-separators used in expert-mode pages, making look&feel more native and similar to other wizards.

FE/Qt: Wizards: Import appliance: Some cleanup.

FE/Qt: Wizards: Export/Import appliance: Few NLS changes.

FE/Qt: Wizards: Make sure expert-mode pages using similar spacings.

FE/Qt: 6123: Expert-mode of import-appliance wizard should fetch passed file-path.

FE/Qt: VBoxDefs renamed to UIDefs and reworked into namespace. Corresponding files which were using VBoxDefs updated.

FE/Qt: Export/Import Appliance wizards: More suitable view-port size for appliance-widgets.

FE/Qt: Wizards rework: 1. Better NLS for all the wizards, 2. Removing (reworking, cleanup required) welcome pages, 3. Removing (hiding, cleanup required) summary pages.

FE/Qt: 6123: Initial implementation of expert-mode for VirtualBox wizards.