Add ability to keep the panel hidden.

[MagneFire]

This adds a toggle that allows you to enable/disable the ability to reveal the panel when it's obstructed by windows.

The default behaviour remains unchanged, only an option is added to keep the panel always hidden when it's obstructed by any window. This is particularly useful when someone wants to click on an item at the bottom of the screen in that case the panel will no longer reveal itself.

It partly implements the suggestion provided in this discussion: #1748

Preview:

[MagneFire]

Here's some more background as to why I decided to implement it as it would appear that the "Require pressure at the edge of the screen to reveal the panel" would solve the ideal solution since it could solve the issue where the panel would appear only when you deliberately want it.

But there's an issue when you have a multiple monitor setup.
Assume the following setup:

• There are two displays. One stacked above the other.
  • An example of this is a typical laptop + monitor solution.
• Set dash-to-panel to be visible on the top edge on the bottom display.
• When the "Require pressure at the edge of the screen to reveal the panel" feature is enabled you will not be able to move the cursor from the bottom display to the top display when you try to move the cursor from the bottom display to the top display as a "barrier" is created preventing the cursor to move across the display.
• The same issue would occur when you use dash-to-panel on the bottom edge of the top display.

This PR works around some of these inconveniences by adding an option to never show dash-to-panel when it's obstructed.