Large file support

[fei0316]

Google Play Comment:

取消建議模式，直接選取臺灣用詞，按轉換，不到2mb的txt，直接崩潰關閉app，有點傻眼…

Idea: do not populate text in TextView on large files and disable clipboard

[sls1005]

If the file is really that large, then we not only should avoid rendering it but also should avoid reading it into the memory wholly at once.

The app should only show the first few lines for preview, and if the user decide to convert it, you read it line by line and convert each line; make sure that the string holding the line is freed / destroyed / out of scope after that, so that it will no longer occupy the memory and the memory could be reused. This algorithm would be slower but consume less memory.