Fixes for interface etc.

Change: Going from WindowState Normal to Minimized and back to Normal no longer removes the last PuTTY row
Change: Close session list after session is opened from list
Change: Session list not shown on open if saved tabs exist
Change: PuTTY panel DockAreas limited to DockAreas.Document
Change: Move native methods to separate class
Change: Change target framework and build files to newer version
New: Additional items in tab right click menu; new session, PuTTY menu items
New: Focusing PuTTY also focus SuperPuTTY
New: Alt + Shift show menu bar
New: Restore window position on open
New: Restore tabs on open
New: Save password
New: Application use icon loaded from putty.exe