pwmgr

Move imgui code to "external" directory.

Setting default focus on filter text box for account screen. Also adding
ImGui example program for reference.

Update Dear ImGui to v1.91.5.

Remove dependence on exernal font file. Used binary_to_compressed_c
utility to convert Roboto Medium font into compressed array so that it
can be included directly within the source code.

Significant improvements including reading input from encrypted file.
Also added crypto utility to en/decrypt a file.

Hasty WIP commit.

Using "gui_test" as a starting point.