Add support for remote debugging with long-living "gdbserver --multi"

**Is your feature request related to a problem? Please describe.**
Remote debugging is almost necessary for working with targets of other architectures. When using gdbserver without "--multi" option, the binary can only be run once. This requires going back and forth to the target device console window to reset and rerun every time.
Currently, when attempting to connect to "gdbserver --multi", there is an error in the gdb console. It suggests I use "extended-remote."

**Describe the solution you'd like**
I would like to be able to "connect to gdbserver" that's running with "--multi" option, then select the remote executable. This would allow me to control the first run and parameters (hopefully) from within gdbgui. I could restart the executable and debug multiple times. All from within gdbgui.

**Describe alternatives you've considered**
I currently work around this by entering a series of commands in the (gdb) console at the bottom of the web gui:
```
target extended-remote :
file
set remote exec-file
break main
run
```
From that point, everything in gdbgui seems to work normally.
Essentially, this feature request is (at a minimum) automating these commands.
If there was a way to do "browse for remote executables" etc, that would be great.

**Additional context**
This is a snip of my current use case:
```
# On Target:
gdbserver --multi :

# Invoke GDBGUI on dev server with cross-compile tools:
gdbgui --remote --gdb

# On GDBGUI (gdb) prompt:
target extended-remote :
file
set remote exec-file
break main
run
```