Fix copy/paste behavior when interacting with terminals

[dymk]

• I have added an entry to CHANGELOG.md, or an entry is not needed for this change

(I'm not sure how you would like changes recorded in the changelog; should a new version number be created?) 

Summary of changes

• Fixed copy/paste into the gdb & program pty terminals. The clipboard APIs being used were outdated and did not work in modern browsers.
• Changed copy/paste hotkeys to ctrl + alt + c / ctrl + alt + v, because ctrl + alt + c is already bound by some browsers to open developer tools, sometimes in a way which cannot be unbound.
• Added a text input field below the terminals which accept user keypresses, for easier copying and pasting with a native widget in a situation where the user wants to use ctrl + c / ctrl + v without those keypresses being captured by the terminal.\

Fixes #374

Screenshots:

Test plan

• Tested ctrl+alt+c with terminal windows focused, observe that the "copied" toast is triggered and the clipboard is changed.
• Tested ctrl+alt+v with terminal windows focused, observe that clipboard contents are pasted into the terminal
• Tested entering / copying / pasting content in the text input below the terminal input, press "enter" key, observe that input is correctly relayed to the pty with proper newline at the end.