String changes, new function, some code rework.

- Fixed some spelling and grammar errors in the 'strings' resource
- Added a new library called "arity" from developer Javia, which is used
to calculate the equation formed from raw user input.
- Added boolean method isASign to assist in making the switch statement
more simplified
- Did code rework for btnLeftP; fixed a potential issue where a syntax
error may be thrown if the left parentheses button is pressed
immediately after the right parentheses button.
- Fixed a potential issue where a syntax error may be thrown if the
right parentheses button is pressed immediately after the left
parentheses button.
- Added code for method solve: uses the arity library to calculate the
equation formed from raw user input and returns the solution in a
string.
- Modified btnEqual to make use of this new code.
- This effectively brings a new function to the calculator: the function
to solve equations.
- Started code rework for btnDot to fix an issue where the decimal may
not show if one operand has a decimal but the other operand does not.