[Avalonia] Improve the menu items hotkey look and feel. Fixes #904

[snechaev]

This PR improves the look and behavior of the main window main menu items visual and behavior:

• InputGesture attributes added to display the associated hotkeys
• manual handling of the menu items hotkeys removed from the main window KeyUp handler to prevent the corresponding actions to be called twice (first time the MenuItem.HotKey calls associated action, then KeyUp calls the same action again). Fixes [avalonia] Build called twice when Shift+F6 hotkey is used #904

[snechaev]

Note: for now, the MainWindow.xaml contains some copy&paste like this

                <MenuItem Name="Open"
                          Header="_Open Log..."
                          HotKey="Ctrl+O"
                          InputGesture="Ctrl+O"/>

(the Ctrl+O constant written twice).

This can be replaced with something like

                <MenuItem Name="Open"
                          Header="_Open Log..."
                          HotKey="Ctrl+O"
                          InputGesture="{Binding HotKey, RelativeSource={RelativeSource Self}}"/>

This is way more wordy, but it prevents non-synchronous changes to the HotKey and InputGesture attributes.
I can update the PR with such a changes if needed.