Cppcheck and clazy warning fixes

[ElTh0r0]

I hope such kind of code quality improvements are wanted, since I'm touching many files with just "minor" changes.
This patch is mainly fixing warnings related to Cppcheck & Clazy passing args-by-ref / args-by-value, old style Qt signal/slots and using variable names same to function names.

[mmahmoudian]

Thanks for the PR. I think this should be discussed. some of the changes seems reasonable to me, but some not so much. For instance:

imho good change:

• void addToNameEditor(QString s); -> void addToNameEditor(const QString& s);

imho not so good change:

• const QString& value = *it; -> const QString& val = *it;

[ElTh0r0]

imho not so good change:

* `const QString& value = *it;` -> `const QString& val = *it;`

reason for above change is that there is a function with name "value". So either the variable or the function name should be changed:

flameshot/src/cli/commandlineparser.cpp

Line 301 in d2273c6

QString CommandLineParser::value(const CommandOption& option) const

I did some more variable renaming due to the same reason in other files.

[veracioux]

@ElTh0r0 Thanks for contributing to Flameshot. Some of them you should revert - I left comments to let you know which. But I very much appreciate you changing the SIGNAL(...) and SLOT(...) syntax. This was something I was going to do but never got around to it.

[ElTh0r0]

@veracioux many thanks for the detailed review! Please check my changes after the last commits.

[veracioux]

@ElTh0r0 Thanks. All looks good now.

[mmahmoudian]

@veracioux should we proceed with the merge?