Tweaks to balance display

* Now displays account number
* Fixed available credit display
Fixed button names on MainForm
Added ModalDialog class

Just a basic form that follows Charlie's rules for Modals.
Dialog additions

* Added DepositOrWithdrawDialog class
* Deposit, withdraw, transfer buttons now launch new dialogs (with no
content yet)
Assortment of small features

* Added methods for withdraw, transfer, and deposit
* Put accounts in deposit dialog combo box
Added Transaction and DepositTransaction

Don't do much yet, pretty basic.