imgui backend (v2)

[griffi-gh]

closes #95

Adds an ImGui-based UI backend.
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
• [ ] 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