Replaced .png drawables with vector assets where possible

[kalina559]

All the icons that could be found in Android Studio clipart gallery have been replaced with their .svg versions. Resolves #701

[hiqua]

We have to check how that works license-wise, given that some of these files are under the Apache license with LHT is under GPL3.

[kalina559]

The ic_notification.xml was actually created by converting the intro_icon_1.png to .svg using an online converter, so I'm not sure which license applies to it.

[kalina559]

And I also found this: https://www.apache.org/licenses/GPL-compatibility.html

[iSoron]

Thank you for the PR, @kalina559. It looks good. I'll merge after the checks are done.

On a side note, I have just realized that many of these icons are no longer used within the app. If you would like to submit a PR removing these unused resources, that would also be very welcome.

[kalina559]

Should I do it in a separate PR or just add it to this one?

[iSoron]

@kalina559 That can be a separate PR. I've just merged this one. Thanks again!