qcalc spawning behavior change

[bootstrap-prime]

I changed the behavior of calc_preprocess_input to only call qcalc when the input string changes. This is a partial solution to #40. I'm still learning C so I may or may not have a better solution for this later on.

[svenstaro]

@bootstrap-prime Are you interested in following up on my little review?

[bootstrap-prime]

absolutely, sorry for the delay.

[bootstrap-prime]

I sort of figured out how to make this better - spawn an interactive qcalc session and then pipe up inputs and outputs.
Unfortunately, it seems that gio g_input_stream_* doesn't have the ability to read, and then return after a timeout, and I'm not familiar enough with async here to implement a cancel.
I thought I could get around this by just directly using libqalculate, but it's C++ and this project is pure C. libqalculate may expose a C abi, but I'm not sure how to use it or what to look for.

[svenstaro]

Shall I take a proper look at this PR or do you wanna research this topic further first?

[bootstrap-prime]

I don't think I have the motivation to continue, honestly. Most of the window opening spam is removed, so this is probably a good stopping point.

[svenstaro]

Sorry for forgetting about this for so long. Finally merged it. Thanks!