Add imgui based gui example

[bjorn3]

It looks much better with a ttf font instead of the builtin imgui font, but I will leave that for a future PR.

[nbaksalyar]

Looks great, thanks! Would you also like to add the font files?

macOS CI run fails with the remote end hung up unexpectedly which seems like a problem on the Travis's end. My vote is for ignoring this temporarily - there will be more PRs dealing with missing features for macOS anyway.

[bjorn3]

I will add a font tomorrow. The travis issue looks like a temporary network outage.

[bjorn3]

New screenshot with FiraMono (171KiB file) as font.

[bjorn3]

---- target::linux::writemem::tests::write_memory_ptrace stdout ----

thread 'target::linux::writemem::tests::write_memory_ptrace' panicked at 'called `Result::unwrap()` on an `Err` value: Sys(EPERM)', src/target/linux/writemem.rs:292:91

I didn't change any of the relevant code, but there also doesn't seem to be a race-condition.

[nbaksalyar]

This seems like a bug in the ptrace tests. I will create a separate issue to tackle it, and I restarted the CI - let's see if it shows up consitently.

[bjorn3]

Happened again.

[nbaksalyar]

And now it's passed. I created #116 and will investigate it as a separate issue.
Merging this PR now.

Thanks!