Placeholders (#74)

* first draft

* refine placeholder logic

bug: inputting and deleting a char on a widget quickly (i think at cursor blink speed) hold the input visually for that blink duration.

needs testing: placeholder specific attrs

* fix conflict with #73

* update placeholder attrs

* small fixes

* fix cursor showing on empty readonly widgets

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Co-authored-by: StaffEngineer <velo.app1@gmail.com>