Add ability to hide titlebar

# This MR adds the option to hide the title bar to provide a seamless visual effect.

![Screenshot_20201116_112104](https://invent.kde.org/utilities/yakuake/uploads/f248b52cfee904f8327d9e5f373a89c0/Screenshot_20201116_112104.png)

## Changes

To facilitate this change the borders also got an option to be hidden:
*Note:* The bottom border also moved from the skins background to `MainWindow::paintEvent`. The color and width are following the specification provided by the skin.

![image](https://invent.kde.org/utilities/yakuake/uploads/267141465e082ff35fbb3c0fc7e8a6f0/image.png)

## Safeguards

To warn the user that they are about to hide the title bar, which is (when no tray icon is displayed) the only option to access the settings (besides from to the user maybe not known keyboard shortcut) and re-show the title bar, a dialog opens when the user presses the shortcut (default: Ctrl+Shift+m following konsole's model) or is about change the setting in the settings window (red circ...