21497 [linux] Allow engine flags via environment vars (affects: desktop, affects: engine, cla: yes, platform-linux)

21612 Separate mutators for text and selection (affects: desktop, affects: text input, cla: yes, platform-linux, platform-windows, waiting for tree to go green)

21663 Store selection base/extent as size_t (affects: text input, cla: yes, platform-linux, platform-windows)

21682 Add multi-step IME support to TextInputModel (affects: text input, cla: yes, platform-linux, platform-windows)

21685 Make TextInputModel::selection_start/end const (affects: text input, cla: yes, platform-linux, platform-windows)

21711 Perform selection check in DeleteSelected (affects: text input, cla: yes, platform-linux, platform-windows)