PC updategui: two new dialogs

Changes to PC-BSD update GUI:
1. Now main window has a main menu
2. Patchset selection dialog (ported from Update center) Details and screenshot are here:  https://bugs.pcbsd.org/issues/5874 (Ticked #5874)
3. System update descriptions dialog (ported from Update Center). Details and screenshot are here:  https://bugs.pcbsd.org/issues/5875 (Ticked #5875)