Rework input blocking for in-game UI panels

[sakura57]

Currently, certain UI panels block game input. For example, opening the sector map will block input, allowing the player to move the cursor around the map and click on stuff without turning the player's ship or firing the guns.

However, the player's ship continues on its original trajectory, and maintains the previous throttle value. This means that when certain menus or windows are open, the player might accelerate to high speeds or drift out of an area when they may not want to.

To-do list:

• Add another alternative to input blocking which pauses the game. UI panels can opt to pause the game entirely while they're open.
• Fix the current input blocking so that the player ship's throttle is zeroed and their ship stops when a window is opened.

Related: #19

[sakura57]

Addressed in #28. Windows that block input now pause the game by default.