Dear Imgui dialog backend for desktop

[rollerozxa]

Tracking issue for the WIP Imgui dialog backend as #138 will soon be merged into master.

Primarily it will be replacing the current GTK3 dialogs on desktop. Maybe replace the native dialogs on Android, but only if it can match the usability of the native Android dialogs (See #95).

Current state of implemented dialogs:

• [X] DIALOG_QUICKADD
• [X] DIALOG_BEAM_COLOR
• [x] DIALOG_SAVE
• [x] DIALOG_OPEN
• [x] DIALOG_NEW_LEVEL
• [x] DIALOG_SET_FREQUENCY
• [x] DIALOG_PIXEL_COLOR
• [ ] DIALOG_CONFIRM_QUIT
• [ ] DIALOG_SET_COMMAND
• [ ] DIALOG_STICKY
• [ ] DIALOG_FXEMITTER
• [ ] DIALOG_CAMTARGETER
• [x] DIALOG_SET_FREQ_RANGE
• [ ] DIALOG_OPEN_OBJECT
• [ ] DIALOG_EXPORT
• [ ] DIALOG_SET_PKG_LEVEL
• [ ] DIALOG_ROBOT
• [ ] DIALOG_MULTIEMITTER
• [ ] DIALOG_PUZZLE_PLAY
• [ ] DIALOG_TIMER
• [ ] DIALOG_EVENTLISTENER
• [x] DIALOG_SETTINGS
• [x] DIALOG_SAVE_COPY
• [x] DIALOG_LEVEL_PROPERTIES
• [ ] DIALOG_HELP
• [ ] DIALOG_DIGITALDISPLAY
• [x] DIALOG_PLAY_MENU
• [ ] DIALOG_OPEN_AUTOSAVE
• [ ] DIALOG_COMMUNITY
• [ ] DIALOG_PROMPT_SETTINGS
• [ ] DIALOG_PROMPT
• [ ] DIALOG_SFXEMITTER
• [x] DIALOG_VARIABLE
• [x] DIALOG_SYNTHESIZER
• [ ] DIALOG_SEQUENCER
• [ ] DIALOG_SHAPEEXTRUDER
• [ ] DIALOG_JUMPER
• [ ] DIALOG_REGISTER
• [ ] DIALOG_PUBLISHED
• [ ] DIALOG_CURSORFIELD
• [x] DIALOG_ESCRIPT
• [ ] DIALOG_ITEM
• [x] DIALOG_SANDBOX_MODE
• [ ] DIALOG_RUBBER
• [ ] DIALOG_SOUNDMAN
• [ ] DIALOG_FACTORY
• [ ] DIALOG_SET_FACTION
• [ ] DIALOG_RESOURCE
• [ ] DIALOG_VENDOR
• [ ] DIALOG_ANIMAL
• [ ] DIALOG_POLYGON
• [ ] DIALOG_KEY_LISTENER
• [ ] DIALOG_OPEN_STATE
• [x] DIALOG_POLYGON_COLOR
• [ ] DIALOG_MULTI_CONFIG
• [ ] DIALOG_EMITTER
• [ ] DIALOG_TREASURE_CHEST
• [ ] DIALOG_DECORATION
• [ ] DIALOG_SFXEMITTER_2
• [ ] DIALOG_PUBLISH
• [x] DIALOG_LOGIN