Theme provider and '-t' and '-c' CLI options

[veeso]

Theme Provider

Description

• Unified config module
• One function to serialize and deserialize configurations to/from toml
• Theme config
• Theme configuration in setup
• Default themes
• Color picker component
• Use theme provider in activities

Type of change

Please select relevant options.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] This change requires a documentation update

Checklist

• [x] My code follows the contribution guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [x] I formatted the code with cargo fmt
• [x] I checked my code using cargo clippy and reports no warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have introduced no new C-bindings
• [x] The changes I've made are Windows, MacOS, UNIX, Linux compatible (or I've handled them using cfg target_os)
• [x] I increased or maintained the code coverage for the project, compared to the previous commit

Acceptance tests

wait for a project maintainer to fulfill this section...

• [x] regression test: configuration read, save, write
• [x] If the theme doesn't exist, default is created
• [x] Running termscp with -t imports the provided theme
• [x] Running termscp with -c starts termscp from setup
• [x] if the theme contains an invalid color, an error is raised