Project restructuring: add tactility-headless (#55)

- Created `tactility-headless` to support ESP32 firmwares that don't require graphics
- `tactility` subproject now contains both PC and ESP32 code (to avoid having to split up `tactility` and `tactility-headless` into separate projects, which would result in a very complex dependency tree)
- `tactility` subproject is now defined as component for ESP32 and as regular module for PC
- Improvements for dispatcher
- Added `project-structure.puml` to docs
- `Gui` service now depends on `Loader` service instead of the reverse
- Added `statusbar_updater` service for updating Wi-Fi and SD card icons

docs/project-structure.puml

@@ -0,0 +1,23 @@
+@startuml
+skinparam componentStyle uml1
+
+[tactility] as t
+note right of t : to build and use graphical apps
+[tactility-headless] as theadless
+note right of theadless : to build and use background services
+[tactility-core] as tcore
+note right of tcore : defines, data types, logging, async, etc.
+
+package "Applications" {
+  [app-sim] as appsim
+  [app-esp] as appesp
+}
+
+note right of appesp : app-esp depends on the board \n projects for configuration
+
+[t] ..> [theadless]
+[theadless] ..> [tcore]
+[appsim] ..> [t]
+[appesp] ..> [t]
+
+@enduml