Various UI Improvements

[c0derMo]

This PR adds various UI improvements to the frontend. These include:

• The inventories no longer adhering to a strict grid. The strict grid, where each row is scaled to the largest inventory in the row greatly wastes space in cases where you have many inventories of varying size (aka if you are the DM). Since this might be a controversial change, there is a setting to restore original functionality in the settings.
• Inventories are now collapsable. To avoid accidentally collapsing inventories, collapsing them is done via an extra button instead of just clicking the title bar.
• Expanding/collapsing items or inventories is animated using a new CollapseTransition.
• Popups have show and hide animations, along with making the background of popups a bit darker and more blurred, to emphasize the popup a bit more.
• TODO: Item additions and removals from an inventory are animated, intended to also give the Drag and Drop feature a bit more visual feedback. Therefore, this PR requires Drag and Drop for Items #205 to be merged first.