Statusbar improvements (#36)

- Added sdcard service: it updates a statusbar icon and it can auto-unmount sdcards that were physically ejected by the user.
- Added `is_mounted` check for sdcard drivers
- Refactored assets: moved them from `tactility-esp/` to `data/`
- Made assets work with sim build
- Refactored wifi statusbar icons
- Refactored wifi_manage app access point icons
- Support not having an initial image icon when registering a new icon in statusbars. When a statusbar icon is added, it is now visible/invisible depending on whether an image was specified.
- Keep track of app memory on app start to find memory leaks (needs fine-tuning in the future)
- `tt_init()` assigns `config_instance` earlier, so it can be used during service init

data/assets_sources/wifi_perm_scan.svg

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