[Windows] Add WS_EX_TOOLWINDOW to notification popups
Problem
Currently, whenever a notification pops up, the notification's window is included in the Alt-Tab list and creates a taskbar button.
- Clicking on a combined taskbar icon for Gajim could do the same
- Trying to Alt-Tab into Gajim could focus the notification, instead of the main window
Solution
By adding the window style WS_EX_TOOLWINDOW (0x80) to the notification popups, no taskbar icon will be created and the notification will not be included in the Alt-Tab list.