Added a new level property: Difficulty
* An enum ranging from -1, 0, 1 (Peaceful, Normal, Hard)
* Default difficulty is Normal; pre-existing levels are Normal by
default per the zero value.
Doodad scripts can read the difficulty via the new global variable
`Level.Difficulty` and some doodads have been updated:
* Azulians: on Peaceful they ignore all player characters, and on Hard
they are in "hunt mode": infinite aggro radius and they're aggressive
to all characters.
* Bird: on Peaceful they will not dive and attack any player character.
Other spit and polish:
* New Level/Level Properties UI reworked into a magicform.
* New "PromptPre(question, answer, func)" function for prompting the
user with the developer shell, but pre-filling in an answer for them
to either post or edit.
* magicform has a PromptUser field option for simple Text/Int fields
which present as buttons, so magicform can prompt and update the
variable itself.
* Don't show the _autosave.doodad in the Doodad Dropper window.
* SDL2 builds of the game now set their app window icon.
* Create/Edit Level window is updated to show a tabbed UI to create a
new Level or a new Doodad. The dedicated main menu button to create a
new doodad (which immediately prompted for its size) is replaced by
this new tab's UI.
* Edit Drawing/Play Level window is more responsive to smaller screen
sizes by drawing fewer columns of filenames.
* Bugfix: the Alert and Confirm modals always re-center themselves on
screen, especially to adapt between Portrait or Landscape mode on a
mobile device.
The title screen is now responsive to landscape mode. If the window is
not tall enough to show all the menu buttons (~600px) it will switch to
a horizontal layout with the title on the left and buttons on the right.
WIP "Story Mode" button that brings up a Level Packs selection window.
