Image sequence detection fails in many cases

The image sequence detection is nice feature but doesn't work in the following cases:

1. File names have no base name:

 e.g. 000.png, 001.png, 002.png, 003.png, 004.png, ...

 Blender generates image sequence like this.

2. File names include multibyte characters:

 e.g. 画像000.png, 画像001.png, 画像002.png, 画像003.png, ...

 Multibyte support is essential for asian users.

3. Sequence has discrete numbers:

 e.g. prof000.png, prof005.png, prof010.png, prof015.png, ...

 I often process the results of numerical simulations as such discrete numbered images.

4. Sequence starts with (power of ten)-1 and drop the first image:

 e.g. img009.png, img010.png, img011.png, ... (Drop img009.png)

5. Sequence ends with power of ten and drop the last image:

 e.g. ..., img098.png, img099.png, img100.png (Drop img100.png)

I attached a patch for this issue. Please check it.